A high-pressure gas discharge lamp comprises a sealed discharge vessel with an inner discharge space. Two electrodes project into the discharge space, arranged at a distance from each other, to ignite an arc therebetween. The discharge space has a filling comprising a rare gas and further ingredients such as metal halides. Discharge lamps are used in the automotive field due to their high efficiency and good radiating properties. While many discharge lamps used for automotive front lighting contain mercury, lately, mercury-free lamp designs have been proposed for environmental reasons.
However, besides mercury, also thorium is present in discharge lamps. On one hand, thorium may be present in the salts contained in the lamp filling, e.g. as thorium-iodide. On the other hand, thorium is commonly present as thorium oxide in tungsten electrodes.
EP-A-1349197 describes a mercury free metal halide lamp for use in an automotive headlight. In order to achieve an enhanced luminous efficiency, a low lamp voltage reduction, light with a chromaticity suitable for an automotive headlamp, and an increased, rapidly rising luminous flux, the amount of first halides containing a scandium halide (mass a) and a sodium halide (mass b) are chosen such that 0.25<a/(a+b)<0.8 and preferably 0.27<a/(a+b)<0.37. A second halide (mass c) is present for providing a lamp voltage in place of mercury in an amount such that 0.01<c/(a+b+c)<0.4, and preferably 0.22<c/(a+b+c)<0.33. The halides are present in the discharge vessel in an amount of 0.005-0.03, preferably 0.005-0.02 mg/mm3 of the inner volume. Additionally, Xenon gas is present in the discharge medium at 5-20 atmospheres cold pressure. Rod-shaped electrodes are provided with a shaft diameter of 0.3 mm or more which may be made of tungsten, doped tungsten, rhenium, a rhenium/tungsten alloy or the like. An outer envelope houses the discharge vessel, which may be hermetically sealed from the outside air or may have air or an inert gas at an atmospheric or reduced pressure sealed therein. In an example, tungsten electrodes of 0.35 mm diameter are provided in a discharge vessel of 34 mm3. The discharge medium contains 0.1 mg of ScI3, 0.2 mg of NaI and 0.1 mg of ZnI2 with Xe gas at 10 atm at 25° C. In a first comparative example with a higher amount of the second halide the amount of halides are 0.08 mg ScI3, 0.42 mg NaI and 0.30 mg ZnI2. In a second comparative example the amount of halides are 0.1 mg ScI3, 0.5 mg NaI and 0.2 mg ZnI2.